1. Field of the Invention
The present invention relates, in general, to axial flow fans for moving air axially by rotating a plurality of identical blades extending from a central hub and, more particularly, to an axial flow fan having a high efficiency and generating low noise because the sweep angle, the chord length and the pitch angle of each of the blades are designed to be harmonized.
2. Description of the Prior Art
As well known to those skilled in the art, an axial flow fan consists of a central hub and a plurality of blades extending from the hub, and moves air axially by means of the rotating blades while being rotated by the rotating force transmitted from a power source to the hub. The axial flow fan serves to blow air forcibly to a heat exchanger so as to promote heat radiation from engine cooling water or air-conditioner coolant that is circulated through the heat exchanger, such as a radiator or a condenser.
Generally, the axial flow fan is provided with a shroud that surrounds the blades and is fixed to a heat exchanger. The shroud serves to guide air moved by the rotation of the blades so as to blow a larger amount of air to the heat exchanger and also is used to support a motor that generates driving force to rotate the blades.
A conventional axial flow fan comprises a central hub connected with the driving shaft of a motor, a plurality of blades extending radially outwardly from the hub, and an outer band to which the peripheral ends of the blades are fixed. The axial flow fan is generally made of synthetic resin and formed into a single body. The fan band allows the blades to be restrained within the fan band by connecting the side edges of the blades, thereby preventing the blades from being deformed.
In the construction of the axial flow fan, the blades are directly concerned with the movement of air. Each of the blades has a streamlined cross section functioning to draw air from the front of the axial flow fan using pressure increase through the pressure face due to the rotation of the blades and to push the drawn air toward the rear of the axial flow fan.
With regard to design for such an axial flow fan, the below-mentioned limitations follow.
Since the axial flow fan may be used to cool a radiator for cooling an engine and a condenser for improving the performance of an air-conditioner, the axial flow fan should generate a sufficient amount of airflow necessary for the cooling while overcoming a drop in positive pressure due to the loads of the heat exchangers. Additionally, since a vehicle is provided with many electronic devices and the devices consume a large amount of energy, a blowing efficiency with regard to the quantity of power that the electric motor of the fan consumes should be high. Further, pursuant to the noise restriction, the blowing noise should be small. Besides, the axial flow fan must be free from being easily damaged while being rotated in a high speed.
Since the blades of the axial flow fan are most important so as to design the axial flow fan to satisfy the limitations, the shape, the chord length and the pitch angle of each of the blades are principal design factors.
In order to satisfy the limitations, various axial flow fans are proposed.
In U.S. Pat. No. 4,569,631, there is proposed an axial flow fan wherein the leading edge of a fan blade has a certain amount of backward sweep angle at the root portion near the hub and a certain amount of forward sweep angle at the tip portion near the band and pitch angles are defined along the radial positions. In U.S. Pat. No. 4,684,324, there is proposed an axial flow fan wherein with respect to the median line of each of the blades that is obtained by joining the points circumferentially equidistant from its leading edge and its trailing edge, a backward sweep angle is formed at the hub side of the blade, a forward sweep angle is formed at the radially outer side of the blade, and a position at which the sweep angle changes from a backward sweep angle to a forward sweep angle, its blade length and its pitch angle are defined. In U.S. Pat. No. 5,273,400, there is proposed an axial flow fan wherein its median sweep angle changes from a backward sweep angle to a forward sweep angle along a outwardly radial direction and its blade length, its pitch angle and its chamber angle are defined. In U.S. Pat. No. 5,393,199, there is proposed an axial flow fan wherein the median sweep angle is a forward sweep angle all along the outward radial direction, the sweep angle does not exceed 15.degree. at the outer end of the blade, a region in which a leading edge line and a trailing edge line are parallel to a radial line exists, and the chord length decreases after it increases along the outward radial direction.
However, the above-described axial flow fans may improve the blowing efficiencies and reduce noises to a certain degree, but their blowing efficiencies are not improved sufficiently due to excessive sweep angle increases and cracks may be generated at their roots.